This invention relates to a low alpha-ray level glass which emits only an extremely low level of alpha-ray and therefore is suitable for use, e.g., as a cover glass used as a semiconductor device constituting material for, e.g., a solid-state image sensing device and also as a sealing glass filler.
Various types of glasses have recently been used as semiconductor device constituting materials. For example, a cover glass is used for protecting a solid-state image sensing device which is used often in a video tape recorder incorporating a camera. For this cover glass, borosilicate glass and alkali-free barium silicate glass which have excellent chemical durability and hardness and are equivalent in coefficient of thermal expansion to an alumina ceramic package material are used.
Recent demand for high resolution and high density semiconductor device materials has turned a soft error caused by alpha-ray which was neglected in the past to an increasingly serious problem. T. C. May et al have clarified that such soft error is caused by alpha-ray (alpha-particle) emitted when alpha-decay takes places in a small amount (in the ppm or ppb order) of natural radioactive elements such as uranium and thorium which are contained as impurities in semiconductor device constituting materials including a cover glass and a sealing glass filter (T. C. May, M. H. Wood; Annu. Proc. Applied Phys. Symp, Vol 16th, Page 33-40, 1978). For this reason, there is an increasing demand for a semiconductor device constituting material emitting a minimum possible amount of alpha-ray. For satisfying this demand, it is conceivable to purify the above mentioned glass materials used for the conventional cover glass to a higher degree by employing a special refining process. This approach however requires a complex manufacturing process and besides is extremely costly and uneconomical.
It is, therefore, an object of the invention to provide a glass which emits only an extremely small amount of alpha-ray which is a main cause of a soft error while maintaining excellent chemical durability, coefficient of thermal expansion and hardness and therefore is suitable for use as a cover glass or a sealing glass filter used as a semiconductor device constituting material.